This invention relates generally to imaging and, more particularly, to registering volumetric images.
A volumetric medical imaging system includes a two-dimensional data acquisition system and generates three-dimensional (3D) images. For example, a known volumetric computed tomography (VCT) imaging system includes two-dimensional arrays of detector elements for gathering attenuation measurements in the x, or "in-slice", direction and in the z, or "slice", direction. The attenuation measurements are collected and processed to generate a 3D image.
In some imaging applications, such as volumetric digital subtraction angiography (DSA) for observing vasculature, two volumetric images are generated from data sets obtained from two scans performed at different times. For example, in one known DSA imaging method, X-ray images of anatomy are taken before and after an X-ray opaque contrast agent is injected into the blood vessels. The X-ray image taken before injecting the contrast agent is sometimes referred to as the mask image and the X-ray image taken after injecting the contrast agent is sometimes referred to as the opacified image. Logarithmic subtraction of the mask image from the opacified image should remove all but the image data associated with the opacified blood vessels.
Volumetric data sets acquired at two different times from the same patient, however, rarely match each other on a point by point basis. Patient motion, including involuntary motion of certain organs, a change in camera parameters as a function of time, and hysteresis in the sensor, i.e. non-repeatability from one scan to the next, are common causes of the data set misregistration. Such misregistration causes image artifacts in, for example, DSA images. The misregistration in the two data sets, however, is a complex spatial function and cannot be corrected by simple techniques such as translation, or rigid-body rotation, of one data set with respect to the other set.
Known attempts to reduce such artifacts include utilizing global transforms, or a transform applied to a subvolume, to generate a "warped" image for subtraction. Such known attempts, however, do not provide a single warping transformation which takes into account local displacement at each match point for an entire volumetric image.